Communion is a tricky subject to teach
children. I think you have to start with your denomination's doctrine first. Once you have figured out what your church believes, then you can begin teaching. Send info (date,
time) and invite parents to voluntarily participate 3-4 weeks in advance.

This is what I do for my kids:
We read 1 Corinthians 11:23-29 (sometimes the kids read depending on who's there that morning). We discuss it using these questions:

What was Jesus doing?
What was he saying to the disciples?
What are we to remember?

Then I discuss three things communion is to remind us of:

Jesus' death and resurrection.
Unity (Being together as one body [of Christ], mind, and spirit)
And Jesus' return (coming back for us!)

Then we make careful note of v.27-29
We discuss something simple like, we shouldn't have bad feelings about someone because of what
he or she did or didn't do to/for us. I give the kids a chance to make sure they are not holding any offenses (once a little boy went next door to the nursery to hug his little brother and forgive him).

I also make them realize the "bread" and "cup" is
not candy--they are not taking it because they are hungry or will love the taste. We only do it because Jesus asked us to remember Him. Many students opt to abstain from partaking when they learn
it is starchy tasting. (This weeds out the children who are ready to learn and those who aren't.) Remember to keep all the kids
together, because the un-ready ones will learn as the rest honor and remember God's sacrifice and promise.

(Also, if you partake at tables you will keep spilling to a minimum.)
Then we partake together as I read the scripture again.

Sometimes (depending on how young the group is) I will take a couple of Sundays of instruction before we actually take communion.

The most important thing to remember is they are learning by doing. They won't get it all the first time (no matter how long you spend teaching),
but as you work with them they will learn more than most of the parents know and have been doing it for most of their adult life. In fact, I have had parents come to our service and remark that they never new communion meant all that. Thank goodness our God is a forgiving and loving God. He so wants us to learn and grow closer to Him. I believe that even if we don't completely understand everything
every time we take communion, He will understand. I have a hard time believing the disciples knew everything he was talking about at the last supper!